¿Puede un único enrutador habilitado para NAT permitir que algunos usuarios usen NAT y otros usuarios en la misma interfaz Ethernet continúen usando sus propias direcciones IP??

Sí. This can be accomplished through the use of an access list describing the set of hosts or networks that require NAT. All sessions on the same host will be either translated or will pass through the router and not be translated.

Access lists, extended access lists, and route maps can be used to define rulesby which IP devices get translated. The network address and appropriate subnet mask should always be specified. The keyword anyshould not be used in place of the network address or subnet mask. With Static NAT configuration, when packet doesn’t matched with any STATIC rule configuration, packet will be sent through without any translation.